Lead containing glazes and enamels for glass substrates are well-known in the ceramic art. A glaze is typically thought of as a clear coating material, whereas an enamel is a glaze which contains a pigment or other particles in suspension. Such glazes and enamels are generally applied to glass surfaces in the form of a paste, the paste containing finely ground particles of glassy material, commonly called "frit," and a vehicle. After application of the glaze or enamel to the glass surface by silk screening or other techniques, the glass is fired to volatilize the vehicle, fuse the frit and bond the glaze or enamel to the glass surface.
The use of lead oxide as a frit ingredient serves to lower the melting point of the frit. Recent efforts have been made at retaining the low melt temperature of these glazes and enamels but with the removal of the lead oxide.
U.S. Pat. No. 4,892,847, issued to Reinherz, relates to lead-free glass frit compositions. The compositions include from 25-35% silicon oxide, 25-45% of bismuth oxide, 10-25% boron oxide, 4-19% alkali metal oxide, and 0.3-8% of ZrO.sub.2 /TiO.sub.2. Reinherz teaches that coatings which significantly exceed the 45 weight percent bismuth oxide concentration exhibit neither excellent color stability when blended with certain pigments in a glaze or enamel nor resistance to scratching.
European Patent Application 370,683, naming Murkens et al. as inventors, relates to a glass flux composition consisting essentially by weight of 45-65% bismuth oxide, 25-36% silicon oxide, 4-6% boron oxide, 3-6% titanium oxide, 1-3% sodium oxide, 0.5-2% potassium oxide, 2-6% lithium oxide, 0-3% aluminum oxide, 0-1% zirconium (IV) oxide, 0-1% barium oxide, 0-1% calcium oxide, 0-1% magnesium oxide, 0-1% zinc oxide, 0-3% lead oxide, and 0-0.5% cadmium oxide.
The article "Unusual Properties and Structure of Glasses in the System Bi.sub.2 O.sub.3 --B.sub.2 O.sub.3 --SrO; Bi.sub.2 O.sub.3 --B.sub.2 O.sub.3 --BaO; Bi.sub.2 O.sub.3 --B.sub.2 O.sub.3 --ZnO; and Bi.sub.2 O.sub.3 --B.sub.2 O.sub.3 --PbO", by Janakirama-Rao, describes various curious bismuth borate-based glasses. Bi.sub.2 O.sub.3 --ZnO--B.sub.2 O.sub.3 glass compositions are described and illustrated in FIG. 1 of this article.